Hitherto, as the element member of semiconductor device, photosensitive device for use in electrophotography, image input line sensor, image pickup device, photoelectromotive force device or other various electronic elements and optical elements, there have been proposed a number of amorphous semiconductor films, for instance, an amorphous silicon compensated with hydrogen or/and halogen (e.g., F, Cl) [hereinafter referred to as "A--Si(H,X)"]. Some of such films have been put to practical use.
These deposited films have been known to be formed by plasma CVD method wherein a raw material gas is decomposed by subjecting it to the action of an energy of direct current, high frequency or microwave to thereby form a deposited film on a substrate of glass, quartz, heat-resistant synthetic resin, stainless steel or aluminum.
Now, recent years, industrial attention has been focused on a microwave plasma CVD method (hereinafter referred to as MW-PCVD method) using the microwave glow discharge decomposition.
One representative known apparatus for the formation of a deposited film by way of MW-PCVD method is such that has a structure as shown by a schematic perspective view of FIG. 3. In FIG. 3, 301 is a reaction chamber having a substantially enclosed structure. 302 is a microwave transmissive window made of an dielectric material (such as quartz glass, alumina ceramics, etc.) that can transmit efficiently microwave into the reaction chamber and can retain the vacuum. 303 is a metallic waveguide to propagate the microwave, and it is connected through a matching box, isolator (not shown) to a microwave power source (not shown). 304 is an exhaust pipe that one end is open to the reaction chamber 301 and the other end is connected to an exhaust apparatus (not shown). 305 is a substrate placed on a substrate holder in which electric heater 307 being provided, on which a deposited film is to be formed, and 306 is a discharge space surrounded by substrates 305.
Deposited film formation according to the conventional deposited film forming apparatus is conducted in the following way. That is, the exhaust valve is opened, and the reaction chamber 301 is evacuated by the vacuum pump (not shown) to adjust the inner pressure in the reaction chamber to 1.times.10.sup.-7 Torr or less. Then, the heater 307 is activated to heat the substrate 305 at temperature suitable for the formation of a deposited film, and the substrate is kept at this temperature. Thereafter, in the case of forming an amorphous silicon deposited film, silane gas, hydrogen gas and like raw material gas are introduced through a gas introducing means (not shown) into the reaction chamber 301. Simultaneously, the microwave power source is actuated to generate the microwave with frequency of at least 500 MHz, preferably 2.45 GHz, and the microwave is introduced through the waveguide and the microwave transmissive window 302 into the reaction chamber 301. Thus, the gas in the reaction chamber 301 is activated and dissociated to form a deposited film on the surface of the substrate 305.
In such conventional deposited film forming apparatus, there often occur problems on the durability of the transmissive window and the transmission efficiency, when introducing microwave into the reaction chamber. Hithereto, there have been used for the microwave transmissive window, materials having a low dielectric constant (E) and dielectrics loss angle (tan .delta.) to prevent the transmission loss as much as possible. Such materials are berylia (BeO), teflon, alumina ceramics, etc. Further, it is required for the transmissive window material to have sufficient resistances to, the discharged heat radiation, to thermal impact and a also to a vacuum retentivity. However, desirable materials having such characteristics have not been found. In fact, in the case of using the conventional microwave transmissive window, the microwave power is introduced continuously at a range of 10 W/cm.sup.2 to 50 W/cm.sup.2 to cause and continue the plasma discharge, and in this case, the window material will be damaged in a short time. That is, the known dielectric material to constitute the microwave transmissive window is not durable enough for continued use. In view of this, there is an increased demand to provide a desirable material which enables the preparation of a microwave transmissive window to meet the above conditions desired therefore and which is sufficiently durable for continued use.